Clean up Nix configuration and remove old files

- Move Nix package and module to nix/ directory for better organization
- Remove old Nix files (systant.nix, systant-old.nix, etc.)
- Remove debug script and systemd service file
- Update config files for new Nix structure
- Add CLAUDE.md with project documentation

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

.gitignore

@@ -21,3 +21,10 @@ system_stats_daemon-*.tar
 
 # Temporary files, for example, from tests.
 /tmp/
+
+# Nix direnv cache and generated files
+.direnv/
+
+# Nix result symlinks
+result
+result-*